1. Field of the Invention
The present invention relates to image forming apparatuses, and more particularly, to an image forming apparatus determining a time of exchange of units based on the condition (the number of uses) of the unit used in image formation.
2. Description of the Related Art
In order to sense the number of uses (lifetime) of consumable units constituting an image forming apparatus such as a copying apparatus and a laser beam printer, conventionally known is a consumable unit (eg. a process cartridge) having a non-volatile memory contained therein such as a non-volatile RAM and E.sup.2 PROM (Electrically Erasable and Programmable ROM) (refer to Japanese Patent Laying-Open No. 58-195854).
In such an image forming apparatus, a central processing unit (hereinafter referred to as "CPU", which also carries out other processings) on the side of a body of the apparatus accesses a non-volatile memory in each consumable unit for every printing to count up a counter in the non-volatile memory. Then, by reading out a count value in the non-volatile memory as required and indicating the count value on a display panel, the number of uses of each consumable unit can be known from the non-volatile memory contained in the unit.
Such a structure makes it possible to detect precisely the number of uses even for a used unit which is mounted to the body of the apparatus after being used once, whereby it is possible to grasp a time of exchange of units.
However, when the CPU accesses a non-volatile memory, a heavy load is applied to the CPU. Therefore, access to a non-volatile memory for every printing decreases the processing efficiency of the CPU.
When an E.sup.2 PROM is used as a non-volatile memory, in particular, the CPU must access the E.sup.2 PROM by serial transmission. Therefore, the above-described problem is significant. Moreover, there are also problems as in the following. More specifically, there is a limitation in the number of accesses in an E.sup.2 PROM, and an E.sup.2 PROM having a higher upper limit value of the number of accesses is more expensive. Therefore, in order to access the E.sup.2 PROM for every printing, it is necessary to use an E.sup.2 PROM having an upper limit value of the number of accesses at least higher than the number of use limit of the consumable unit. As a result, an inexpensive E.sup.2 PROM cannot be used.